Stop motion for shifting shuttle box looms



April 3, 1956 A. E. BUCHANAN m, ETAL 2,740,432

STOP MOTION FOR SHIFTING SHUTTLE BOX LOOMS Filed May 4, 195:5 a Sheets-Sheet 1 PATTERN MECHA N \5M ANDREW E. BucHANANJIL ana BRADY C. BQGER, .Ta.

ATTORNEYS INVENTORS.

April 1956 A. E. BUCHANAN m, ET AL 2,740,432

STOP MOTION FOR SHIFTING SHUTTLE BOX LOOMS 3 Sheets-Sheet 2 Filed May 4, 1953 3 m 0 Maw WON .W 4 3 4 6 70 .C 2 6 m 2 3 l I EY w wl 8 9 w M .,I 9 ma 6 w \\Hll\lfil|ll\-| u H m R J a .iLlllllm Y 9 m m 64 5% J a 88 7 0 1% L. 2 m w M 7 W o m 2 e f 1. g 7 m e 7 %6 uh v, 7 7 m J\ w, 7 0 IE we & W 7 5 7 mm 4 M ATTORNEYS April 3, 1956 A. E. BUCHANAN m, EIAL STOP MOTION FOR SHIF'IING SHUTTLE BOX LOOMS Filed May 4, 1953 3 Sheets-Sheet C5 ATTORNEYS STDP MDTEQN FOR "SLHFTING SHUTTLE BOX LDOMS Andrew E. Buchanan HI, Charlotte, and Brady C. Boger, Jr Raeford, N. (3.; said Roger, Jr assignor to said Buchanan HI Application May 4, 1953, Serial No. 352676 7 Claims. (Cl. 139-336) This invention relates to looms and :more especially to an improved stop motion controlled by the multiple shuttle boxes in the course of shifting of the shuttle boxes and it is the primary object of this invention to provide improved means to efieet loom stoppage in the'event of the shift-ing of the shuttle boxes being prevented for any reason, as by an improperly boxed shuttle.

As is well known to those familiar with the art, shuttle boxes are shifted between bottom and top center positions of the lay of the loom by a pattern mechanism having box chains, belts or cables extending therefrom to a yieldable coupling, such :as a compression spring surrounding .a guide rod or lifter rod whose upper portion supports a tier of shuttle boxes. If a shifting operation is called and a shuttle is notentirely boxed, but overhangs part of the lay, the shuttle boxes will be held against motion but the yieldable coupling and otheroperating mechanisms of the boxes will move. The coupling yields to avoid breakage, but the shuttle remains in improper position and may cause the picker stick to break or another shuttle may be thrown against the shuttle previously improperly boxed, or the improperly boxed shuttle may cause breakage of a substantial number of the warp yarns adjacent the corresponding selvage of the fabric being woven, the latter normally being termed a shuttle smas unless the loom is stopped before the lay has fully advanced to its beat-up position.

Now, since each of the multiple shuttle boxes or tiers of shuttle boxes is raised and lowered by means of a belt, chain, or other flexible element, it is evident that the box chain will be maintained under an optimum tension at all times during the course of normal and proper shifting of the corresponding multiple shuttle box. On the one hand, if the multiple shuttle box is not raised to the proper position in the course of a lifting movement thereof, as effected by the box chain, the box chain will be placed under excessive stress and, on the other hand, if the multiple shuttle box is not lowered to the proper operating position in the course of downward shifting movement thereof, as effected by the box chain, the box chain will then be unduly slackened.

It is therefore another object of this invention to provide means for automatically stopping the loom in the event of the amount of tension in the box chain varying relative to the optimum.

More specifically, it is an object of this invention to provide an improved limit switch mechanism interposed in the box chain; that is, to opposite ends of which the proximal ends of sections of the box chain are connected, and which switch mechanism has a switch therein which is normally open when the two sections of the box chain are maintained under optimum tension, but which switch will be closed upon the amount of tension in the two sections of the box chain being increased or decreased substantially above and below, respectively, the desired optimum tension in the two sections of the box chain.

It is still another object of this invention to provide United States Patent 6 F Patented Apr. 3, 12956 an improved switch mechanism to which the proximal ends of sections of a pattern controlled box chain .for the multiple shuttle boxes of a loom are connected and wherein the switch mechanism has a switch therein which is normally maintained in open position'to thereby break the circuit to the conventional stop motion of the loom, but which, upon abnormally .high tension in the chain sections or the tension in the chain sections being relaxed, will close to thereby energize the conventional stop motion of the loom for stopping the same.

This improved switch mechanism comprises a housing having end walls through which respective links or connecting rods loosely extend, the proximal portions of the connecting rods each having a switch engaging means thereon disposed adjacent a normally open switch common 'to both of the switch engaging means and wherein the first of the connecting rods is yieldably urged toward switch closing position. The second of the connecting rods is yieldably urged to switch opening 'position, but under a substantially greater pressure than that at which the first of the connecting rods is urged to switch closing position. The yielding means associated with the two connecting rods is so arranged that, upon an optimum amount :of pulling forceor tension being applied to the distal ends of the connecting rods, the switch engaging means of the first of the connecting rods will be held out of engagement with the switch to permit the same *to remain open. f course, upon the pulling force on opposite ends of the connecting rods being relaxed, the switch engaging means on the first of the connecting rods will be moved to operative position for closing the switch and, conversely, upon the pulling force on opposite ends of the connecting rods being substantially increased above the desired optimum pulling force, then the second of the connecting rods will be moved outwardly relative to the housing to thereby move its switch actuating means to operative or switch closing position to also close the switch.

it is thus seen that a substantial variation in the amount of pulling force exerted on opposite ends of the connecting rods in either direction will close the switch, and the switch will remain openas long as the pulling force on opposite ends of the connecting rods remains constant and within predetermined limits.

Some of the objects of the invention having been stated, other objects will appear as the description proceeds, when taken in connection with the accompanying drawings in which Figure 1 is a somewhat schematic front elevationshowing the frame of a loom partially broken away, the lay and the multiple shuttle boxes mounted on opposite ends of the lay and also showing a pattern mechanism and a stop motion in association therewith schematically;

'Figure 2 is an enlarged elevation looking at the lefthand side of one of the improved limit switch mechanisms shown in the upper left-hand portion of Figure 1, substantially along line 2-2 but omitting the cover and showing the switch mechanism in normal operating position, wherein opposite ends of the connecting rods of said switch mechanism are under optimum tension, and also showing the switch therein in cross-section;

Figure 3 is a vertical sectional View taken substantially along the line 3-3 in Figure '2 and wherein the cover :is also secured to the housing of the switch mechanism;

Figure '4 is a top plan view of the switch mechanism, partially in section, looking down :along line 4-4 in Figure 2;

Figure 5 .is a wiew similar :to'Figure 12, but showing the position of the parts of the improved switch mechanism when the pulling force on PPOSite :ends of the connecting rods is below a desired optimum;

Figure 6 is a fragmentary vertical sectional view taken substantially along line 66 in Figure and also showing a portion of the cover secured to the housing of the switch mechanism;

Figure 7 is a view similar to Figure 5, but showing the position of the movable parts of the switch mechanism when the pulling force on opposite ends of the connecting rods of the switch mechanism is above the desired optimum;

Figure 8 is a vertical sectional view taken substantially along the line 8-8 in Figure 7 and also showing the cover fixed to the housing of the switch mechanism;

Figure 9 is a sectional plan view taken looking down along line 9-9 in Figure 5;

Figure 10 is a sectional plan view looking down along line 1010 in Figure 5.

Referring more specifically to Figure l of the drawings, a loom is shown somewhat schematically which is provided with multiple shuttle boxes on opposite ends of the lay thereof substantially of the type disclosed in United States Patent No. 397,074 of January 29, 1889. The numerals 10 and 11 indicate the side frame members of the loom, 12 indicates the arch of. the loom, and 13 indicates the lay, which is caused to oscillate back and forth by conventional means, not shown, and is fixed on the upper ends of swords 14 and 14a whose lower ends are fixed on a rocker shaft 15.

Fixed to opposite ends of the rocker shaft 15 are rocker arms or brackets 16 and 16' which pivotally support the lower ends of conventional picker sticks 17 and 17' which are driven in a conventional manner for alternately propelling an active shuttle across the lay v 13, only one shuttle being shown and indicated at S in Figure 1.

Mounted for vertical movement on opposite ends of the lay 13 are respective gangs of shuttle boxes, multiple shuttle boxes or shuttle box assemblies 20 and 20 each of which, in this instance, includes four shuttle boxes.

The shuttle boxes associated with the gang or shuttle box assembly 20 are indicated at 5-11, 8-2, 8-3 and S4 and the shuttle boxes of the gang or shuttle box assembly 20' are indicated at 8-1, 8-2, 8-3 and 8-4.

It is to be understood that only a single shuttle box may be provided at one end of the lay and a plurality of shuttle boxes at the other end of the lay, or the number of shuttle boxes in each of the shuttle box assemblies or gangs 20 and 20' may vary, as desired. The shuttle boxes are under the control of a pattern mechanism which is only shown schematically, since pattern mechanisms are well known, it being deemed sufiicient to state that the pattern mechanism 25 has conventional means therein for reciprocating a pair of wheels or sprocket wheels 26 and 26' for raising and lowering the respective shuttle box assemblies 20 and 20' according to a preconceived pattern.

Heretofore. a box chain has been provided for connecting sprocket wheels 26 and 26' to yieldable couplings 'in raising and lowering the shuttle boxes at opposite ends of the lay 13, as exemplified in United States Patent No. 240,009 of April 12, 1881. In the present instance, each box chain is formed in at least two sections and the chain sections which engagethe respective sprocket wheels 26, and 26' are indicated at 27 and 27. The lower ends of the chain sections 27 and 27' are connected to respective limit switch assemblies or mechanisms generally designated at 30 and 30'. These switch mechanisms 30 and 30 form the nucleus of the present invention and will be later described in detail. The lower ends of the switch mechanisms 30 and 30 are connected to the upper ends of respective lower chain sections 31 and 31' which pass beneath and partially around respective pulleys or sprockets wheels 32 and 32 and then extend upwardly and are connected to respective cables 33 and 33'.

The cables 33 and 33' then extend upwardly and partially around respective pulleys 34 and 34' which are suitably rotatably supported by the lay 13. The cable 33' then extends across the loom beneath the lay 13 and passes over and partially around another pulley 36 suitably rotatably supported by the lay 13. The free or distal ends of the cables 33 and 33' are connected to respective conventional sleeves 37 and 37, by means of respective links or rods 41 and 41'. The lower portions of the sleeves 37 and 37' are supported for vertical sliding movement in the respective rocker arms or brackets 16 and 16'. The lower portions of the shuttle box lifter rods 41 and 41 are slidably mounted in the respective sleeves 37 and 37 and the medial portions of the lifter rods 41 and 41' are surrounded by respective compression springs 42 and 42' whose upper ends bear against the lower surfaces of respective litter forks 43 and 43' in which the upper ends of the respective litter rods 41 and 41' are fixedly mounted.

It is thus seen that, with upward movement of either of the upper box chain sections 27 or 27', the corresponding shuttle box assembly 20 or 20 is caused to move upwardly and, conversely, with downward movement of either of the upper chain sections 27 or 27', the corresponding shuttle box assembly 20 or 20' is permitted to move downwardly. This shifting of the shuttle box assemblies 20 and 20 occurs between bottom and top center positions of the lay 13 and, if a shuttle is not entirely boxed, but overhangs part of the lay 13, theshuttle boxes will be held against motion, but the corresponding box chain will move by virtue of the compression springs 42 and 42 to thereby avoid breakage of the box chain and the intelvening connections between the box chains and the shuttle box assemblies.

However, the shuttle remains in improper position and the picker stick will be broken unless the loom is stopped before the picking point is reached in the m cycle. Also, in some instances, the box chains may break or the gears in the head motion or pattern mechanism 25 may break or, at least, during the course of a beat-up stroke of the lay, when the shuttle is only partially boxed, shuttle smashes are brought about and, hence, the improved tension limit switch mechanisms 30 and 30 are provided in order to overcome such defects.

The switch assemblies 30 and 30 are provided for use with looms having conventional electrically operated automatic stop motion mechanisms thereon and the illustration in the upper left-hand portion of Figure lvshows such a stop motion, schematically, for controlling an electric motor 50 which drives the loom. The electric motor 50 has wires 51 and 52 conected to opposite sides thereof whose distal ends are connected to a plug 53 and a relay switch 54, respectively. The plug 53 may be connected to a suitable source of alternating current, not shown, and also has a wire or conductor 55 extending therefrom to the opposite side of the relay switch 54, which relay switch is normally closed, thus energizing the electric motor 50.

The relay also includes a magnet 56 having wires or conductors 57 and 60 extending therefrom and the end of wire 60 remote from the magnet 56 may be connected to a suitable source of direct current, such as a secondary coil of a transformer, and is shown as being connected to a storage battery 61 which is grounded at 62. The wire 57 has a wire or conductor 57' connected intermediate the ends thereof and the wires 57 and 57 are connected to the respective limit switch mechanisms 30 and 30' limit switch assembly 30 and its function will be described in detail.

Referring to Figures 2 to 20, inclusive, the improved tension limit switch mechanism 3.0, which is broadly a yieldably extensible element, is in the form of a substantially rectangular frame or housing including spaced side walls 65 and 66 whose upper and lower ends are connected to respective end walls 67 and 68 and whose medial portions are connected to oposite ends of a partition 69. The frame or housing is closed by front and rear Walls, plates or covers 72 and 73 whose outer edges preferably conform to the outer surfaces of the frame members or walls 65 to 68, inclusive, and which are suitably removably secured .to the side 'walls '65 and 66, as by screws 74.

A normally open sensitive switch 75 is :suitably secured to the inner surface of the front wall 72, at a point between the partition 69 and the end wall 67, as by screws 76. The switch 75 has terminals 77 and 78 to which corresponding ends of respective wires '82 and 83 are connected. The end of wire 82 remote from the terminal 77 is connected to a terminal 84 which extends through, but 'is suitably insulated from, the side wall 66 and to which the end of wire 57 remote from the relay magnet 56 is connected. The end of wire 83 remote from the terminal '78 is grounded on the side wall 65.

There are various types of switches which may serve the purpose of the switch 75, one being of a type such as is shown in Figure 3 of Patent No. 2,594,090 issued to Clarence B. Stadum on April 22, 1952. Therefore, only so much of the switch 75 "will be described as is necessary to a clear understanding of the present invention.

The housing of switch 75 is preferably made from an insulation material, such as a molded plastic material, and one end of a leaf spring contact member '85 is -fixed to the terminal 77 within said housing, and the other end thereof is normally spaced from the inner end of the terminal 78. A resilient switch actuating plunger 86 is mounted for sliding movement in the housing of the switch 75 and extends upwardly therefrom in Figures 2, 3, 5, 6, 7 and 8, this plunger 86 normally being urged outwardly or upwardly by the leaf spring member 85. It is evident that, when the plunger 86 is moved'inwardly, this moves the leaf spring contact member 85 into engagement with the terminal 78 to thereby close the circuit to the relay magnet 56 (Figure 1), which will cause the relay switch 54 to move to opened position to thus stop the electric motor 50 and, of course, the loom.

Now, the proximal ends of the chain sections 27 and 31 are pivotally or otherwise connected to the distal ends of respective connecting rods or plungers 90 and 91 which loosely penetrate the respective walls 67 and 68 of the switch mechanism 30 and have respective transverse bars 92 and 93 fixed on the inner or proximal ends thereof, which bars are loosely mounted between the walls 65, 66, 72 and 73 of the frame of the switch mechanism 30. Suitable inward-movement-lirniting stop members are provided in the form of projections 94 and 95 extending inwardly from the respective walls 65 and 66 and which are spaced from the wall 67 to thereby limit inward move ment of the transverse bar 92.

Suitable outWard-movement-limiting stop members in the form of projections 96 and 97 are spaced from the partition 69 for limiting outward movement of the transverse bar 93. These stop members 94 to 97, inclusive, are merely provided to prevent damage to the switch 75, since the transverse bar 92 has a switch engaging block 100 extending inwardly therefrom which is normally spaced from, but is adapted to engage and move, the plunger 86 to switch closing position, as shown in Figures and 6. Also, the transverse bar 93 has the lower end of an inverted substantially L-shaped switch engaging member 101 suitably secured to the same or integral therewith, the vertical leg of which loosely penetrates the partition 69 and the horizontal leg of which is adapted to, at times, move into engagement with, and depress, the switch actuating plunger '86 substantially as shown in Figures 7 and 8.

When the switch mechanism 30 is not in use, or the box chain sections 27 and 31 are unduly slackened, the transverse bar 92 is urged to operative position against the stop members 94 and and the switch engaging block thereon is pressed against the plunger '86 of the switch 75, as by a tension spring 103, one end of which is connected to a spring anchor 104 extending from the inner surface of the transverse bar 92 and 'the other end of which is connected to a spring anchor 105 disposed closely adjacent the wall 73 and being suitably secured to the partition 69.

Contrary to the switch actuating member 100, the switch actuating member 101 is normally urged out of engagement with the plunger 86 of the switch 75 by means of a compression spring 106 which surrounds the inner portion of the connecting rod or plunger '91 and one end of which bears against the transverse bar 93 and normally urges the same into engagement with the surface of the partition 69 remote from the switch 75. The end of the compression spring 106 remote from the transverse bar 93 may bear directly against the wall 68, but, in order to facilitate adjustment of the amount of pressure exerted by the compression spring 106, the end thereof remote from the transverse bar 93 bears against a plate 107 which is loosely penetrated by the connecting rod 91 and one end of which bears against an abutment or projection 110 at the juncture of the walls 66 and '68 and the other end of which bears against the inner end of an adjustment screw 111. The adjustment screw 111 threadably penetrates the wall 68 and is held in adjusted position by any suitable means such as 'aloc'k nut 112.

'It is evident that the compression spring 106 necessarily exerts a substantially greater pressure against the transverse member 93 than the amount of inward pressure exerted on the plate or transverse bar 92 by the tension spring 103 so that, when the box chain sections 27 and 31 are under normal operating or optimum tension, the parts occupy the, position shown in Figures .2 and 3, wherein the outer surface of the transverse bar 92 'bears against the inner surface of the wall 67 and the transverse bar 93 bears against the transverse partition 69. Of course, in this instance, the switch actuating members 100 and 101 are spaced from or barely touch the outer end of the switch actuating plunger'86.

Now, when the pattern mechanism 25 (Figure 'l') operates to lower the box chain sections 27 and 31, with clockwise movement of the sprocket wheel 26, the shuttle 'box assembly or gang 20 moves downwardly by gravity. 'It is evident that, if the shuttle is improperly boxed when directed into one of the three lower shuttle boxes 8-2, S-3, or 8-4, this prevents the lowering of the shuttle box assembly 20 to the proper elevation for alinement with the upper surface of the lay 13, with the result that the box chain sections 27 and 31 are slackened and the tension spring 103 causes relative movement between the frame of the limit switch mechanism 3'0 and the transverse '-bar 92, causing the bar 92 to move from the position shown in Figures 2 and 3 to the position shown in Figures 5 and 6. As heretofore stated, the switch actuating member 100 of bar 92 will then close the switch 75 to stop the loom.

Upon the shuttle box assembly 20 being raised, by virtue of the pattern mechanism 25 imparting movement to the sprocket wheel 26 in a counter-clockwise direct-ion in Figure 1, the shuttle box assembly 20 is moved upwardly yieldably by virtue of the compression spring '42 and an improperly boxed shuttle restrains the shuttle box assembly 20 from upward movement, although the box chain sections 27 and 31 may continue to move-upwardly. It is evident that the amount of tension 'on the box chain sections 27 and 31 is substantially increased due to the additional pressure exerted thereon by the compression spring 42, thus exerting a substantially greater than normal or optimum tension or pulling force on the distal ends of the connecting rods W and 91.

It is evident that when this pulling force exceeds the pressure exerted by the compression spring 106, the connecting rod 91 and transverse bar 93 move relative to the frame of the limit switch mechanism 30, from the position shown in Figures 2 and 3 to that shown in Figures 7 and 8. In so doing, the lateral leg of the substantially L-shaped switch actuating member 101 depresses the switch actuating plunger 86 to close switch 75 and thereby stop the loom.

As heretofore stated, the amount of pressure exerted by the compression spring 106 on the transverse bar 93 may be varied by adjustment of the screw 111 in the wall 68.

It is thus seen that we have provided an improved stop motion actuating mechanism for looms provided with an electrically operable Slop motion which automatically stops the loom upon either of the shuttle box assemblies 20 or 2i) being improperly positioned for any reason whatsoever in the course of a box shifting operation such as by an improperly boxed shuttle, since it is evident that the box chain sections 27 and 31 or 27' and 31' are maintained under optimum tension at all times during normal and proper operation of the shuttle box assemblies 20 and 20' and, upon the tension in the chain sections being either decreased or increased the loom is immediately stopped.

Although the stop motion shown in the upper left-hand portion of Figure 1 indicates that the electric motor is stopped upon actuation thereof, it is evident that the limit switch mechanisms 36 and 3% may be employed in energizing any of many difierent types of conventional stop motion mechanisms such as that usually normally actuated by drop wires. As a matter of fact, the wires 57 and 57 in Figure 1 may be employed to energize the solenoid of an electric stop motion of the type disclosed in a patent to Brooks et al., No. 2,300,098 of October 27, 1942. The stop motion mechanism shown in the upper portion of Figure 1 is exemplary and is not to be construed as a limitation.

In the drawings and specification there has been set forth a preferred embodiment of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being defined in the claims.

We claim:

1. In a loom having an automatic stop motion mechanism, a lay provided with two gangs of shuttle boxes with shift during a shifting period preceding a picking operation, a pattern mechanism, two box chains connected with said pattern mechanism and yieldable couplings between each box chain and the gauge of shuttle boxes normally maintaining the box chains under optimum tension; the combination of yieldably extensible elements interposed in said box chains, means connecting the extensible element with said stop motion, means responsive to said extensible element being under a predetermined tension for preventing operation of the stop motion, and means responsive to variations in the amount of tension in either of the box chains independently of the other box chain to a greater and a lesser degree than said predetermined tcnsion, resulting from an improperly boxed shuttle, for actuating the automatic stop motion, the foregoing system atfording operation of the shuttle boxes independently of each other or in unison, the loom stopping automatically any time any shuttle box in either gang is restrained from vertical movement in either direction.

2. In a loom having an electrically operable stop motion mechanism, a lay providedwith a gang of shuttle iii boxes at each end thereof which shift during a shifting period preceding a picking operation, a pattern mechanism, two box chains connected with said pattern mechanism and a yieldable coupling between each box chain and a gang of shuttle boxes normally maintaining the box chains under optimum tension; the combination of a limit switch mechanism interposed in each box chain and including a normally open switch electrically connected with said electrically operable stop motion, and means responsive to variations in the amount of tension in either of the box chains independently of the other box chain from a given optimum tension for closing the corresponding normally open switch to thereby actuate the electrically operable stop motion, the foregoing system affording operation of the shuttle boxes independently of each other or in unison, the loom stopping automatically any time any shuttle box in either gang is restrained from vertical movement in either direction.

3. An improved tension switch mechanism for controlling an electrical circuit in accordance with variations in tension in an operating cable, and wherein means are provided to normally maintain said cable under a predetermined tension, comprising an elongated frame having spaced end walls, first and second connecting rods loosely penetrating the respective first and second end walls and being connected to complementary spaced apart intermediate points of said cable, a switch carried by said frame and disposed between said end walls and being interposed in said circuit, a first switch engaging element carried by the first connecting rod, 21 first spring means urging the first switch engaging element into switch engaging position and wherein said first engaging element is held out of switch engaging position when the cable is under said predetermined tension, a second switch engaging element carried by the second connecting rod, a second spring means having a substantially greater inherent force than the first spring means, said second spring means being so arranged as to urge the second switch engaging element out of switch engaging position when said cable is maintained under said predetermined tension whereby, upon the tension in the cable being relaxed, the first spring means will cause the first switch engaging element to engage said switch and whereby, upon the tension in the cable being increased substantially above said predetermined tension, the second spring means will permit the second switch engaging element to move to switch engaging position.

4. An improved switch mechanism for controlling an electrical circuit in accordance with variations in tension in an operating pliable element, and wherein means are provided to normally maintain said pliable element under a predetermined tension, comprising a frame having spaced first and second end members, a switch carried by said frame and disposed between said end members and being interposed in said circuit, first and second switch engaging elements movably mounted in said frame between said members, means connecting the first and second switch engaging elements to complementary spaced apart intermediate points of said pliable element, a first resilient means urging the first switch engaging element into switch engaging position and wherein the first switch engaging element is held out of switch engaging position when the operating pliable element is under said predetermined tension, a second resilient means having a substantiially greater inherent force than the first resilient means, said second resilient means being so arranged as to urge the second switch engaging element out of switch engaging position when said pliable element is maintained under said predetermined tension whereby, upon the tension in the pliable element being relaxed, the first resilient means will cause the first switch engaging element to engage said switch and whereby, upon the tension in the pliable element being increased substantially above said predetermined tension, the second resilient means will permit the second switch engaging element to move to switch engaging position.

5. A structure according to claim 4 having means limiting movement of said switch engaging elements relative to said frame to thereby prevent said switch engaging elements from exerting excessive pressure on said switch upon the tension in said pliable element being varied substantially with respect to said predetermined tension.

6. A structure according to claim 4 wherein said frame includes a transverse member spaced between said end members, said second switch engaging element comprising an elongated rod loosely penetrating the second end member and being connected to the corresponding inter mediate point of said pliable element, a transverse bar normally engaging said transverse member and mounted on the inner end of said rod, said second resilient means being in the form of a compression spring surrounding the medial portion of said rod and bearing against said transverse bar, a plate disposed inwardly of and closely adjacent said second end member and being engaged by the end of said compression spring remote from said transverse bar, and means for adjusting said plate relative to said frame to thereby vary the amount of pressure exerted by the compression spring.

7. A structure according to claim 6 wherein said means for adjusting said plate comprises a projection carried by the second end member of said frame and disposed on the inner surface thereof and against which one edge of said plate is positioned, and an adjustment screw threadably penetrating the second end member and engaging the end of said plate opposite from the end thereof which engages said projection.

References Cited in the file of this patent UNITED STATES PATENTS 793,775 Esser July 4, 1905 1,246,471 Rumpf Nov. 13, 1917 2,234,619 Bishop Mar. 11, 1941 2,462,041 Hohler Feb. 15, 1949 2,512,055 Dillon June 20, 1950 2,642,097 Wright June 16, 1953 2,706,755 Krasno Apr. 19, 1955 

